This invention relates to a dual layer photoreceptor for use in electrophotography having a high sensitivity upto the near infrared wavelength region.
Heretofore, inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have widely been used for photoreceptors for electrophotography.
Moreover, organic photoconductive materials represented by polyvinylcarbazole have been investigated as well to be applied to the photoreceptors for electrophotography, and several of them have already been put into practical use.
The organic photoconductive materials have various merits, for example, in that they are reduced in weight, can be formed into films without difficulty and facilitate the production of the photoreceptor as compared with the inorganic materials.
Along with the recent prevalency of laser beam printer or the like that uses a laser beam as light source instead of conventional incandescent light and has advantages in higher printing speed, higher picture quality and non-impact printing, development for photosensitive materials capable of satisfying the requirements therefor has been demanded.
A semiconductor laser has been remarkably developed in recent years among the laser beams. In this case, since the wavelength of the semiconductor laser is around 800 nm, a photoconductive material of a property highly sensitive to the long wavelength ray around 800 nm has strongly been desired.
As the organic substance capable of satisfying the above-mentioned requirement, there have been known squaric acid methine dye, indoline dye, cyanine dye, pyrylium dye, polyazo dye, phthalocyanine dye, naphthoquinone dye and the like. However, at the present stage, the squaric acid methine dye, indoline dye, cyanine dye and pyrylium dye are insufficient in the practical stability, i.e., cyclic characteristics although they are adaptable to the longer wavelength, the polyazo dye is less adaptable to the longer wavelength and disadvantageous in view of the production thereof and the naphthoquinone dye is disadvantageous in view of the sensitivity.
The photoreceptor composed of a metal phthalocyanine compound among the phthalocyanine dyes, has a sensitivity peak at a relatively longer wavelength region of from 700 to 750 nm, while it somewhat varies depending on the central metal, as described in U.S. Pat. No. 3,357,989, Japanese Patent Application Laid-Open No. 11136/1974, U.S. Pat. No. 4,214,907 and British Patent No. 1268422, etc. However, the sensitivity is gradually reduced in excess of 750 nm, and turned into no more practically effective level.
Japanese Patent Application Laid-Open No. 49544/1984 describes an electrophotographic photoreceptor in which titanium phthalocyanine is vapor-deposited on a substrate to prepare a charge generation layer, and a charge transport layer mainly composed of 2,6-dimethoxy-9,10-dihydroxyanthracene is prepared by coating. However, this photoreceptor material has a high residual potential to suffer from an unfavorable restriction to some extent in the practical use, and is less advantageous in view of the reproducibility of various electric properties due to the unevenness in the film thickness formed by the vapor-deposition. Furthermore, it bears some inevitable restrictions in the mass production of the photoreceptor in industrial scale. On the other hand, problems are resulted to the photoreceptor itself such as occurrence of interference fringe mainly attributable to the reflection of the laser beam on the substrate at the time of the exposure, while several technics have been known as the countermeasures therefor.
As one of such measures, there has been known a method of increasing the thickness of the charge generation layer to absorb the exposing laser beam, thereby eliminating the reflection from the substrate. But there is a limit to the thickness of the film that can be formed by the conventional vapor-deposition process and the thickness control is also difficult.
On the contrary, a method of preparing the charge generation layer by coating pigment dispersed binder solution is advantageous since this method can prepare the layer with an optional thickness at good reproducibility under easy control, requires no high vacuum system for the vapor-deposition, can avoid heat decomposition or degradation while heating, and additionally it is free from cumbersome procedures in the industrial production such as crystallization of vapor-deposited products in various ways after vapor-deposition as in the vapor deposition process.
The object of the present invention is to provide an organic photoconductive material capable of producing the photoreceptor having a high sensitivity at around 800 nm, while having a desired charging property, extremely low residual potential and satisfactory durability.